[OSGeo-Standards] Web Feature Service (WFS) and Axis Order

Wed Oct 19 16:09:11 PDT 2016

Thanks Carl that is a great link. Can I confirm which "GeoServer" axis
order page you wish corrected?
- http://docs.geoserver.org/latest/en/user/services/wfs/basics.html
- http://docs.geotools.org/latest/userguide/library/referencing/order.html

We have a release coming up and I would love to catch any corrections (and
link to http://www.ogcnetwork.net/axisorder).

--
Jody Garnett

On 19 October 2016 at 11:21, Carl Reed <carl.n.reed at gmail.com> wrote:

> Dear OSGeo community -
>
> I have subscribed to the GeoServer list but have not yet been approved so
> I am posting to this list.
>
> As you may know years ago I facilitates a lengthy and at time lively OGC
> discussion on Axis Order and CRS. The result of these discussions was OGC
> member approval of the OGC Axis Order policy and guidance (
> http://www.ogcnetwork.net/axisorder).
>
> Given this background, Esri recently approached me about some ambiguities
> and confusions regarding the history and evolution of how axis order was
> expressed in a Web Feature Service implementation instance. After review of
> various OGC WFS documents and discussions with the Editor (Peter Vretanos)
> I compiled the following information. I will be sharing the same
> information with the OGC Architecture Board and staff. Apparently, the CRS
> axis order issue never really "dies"!!
>
> I bring this information to your attention so that perhaps the GeoServer
> community can update their WFS Axis Order web page content to be consistent
> with the following. Any questions in that regard, please let me know. Onto
> the discussion.
>
> Part of the confusion is probably due to changing OGC policies over the
> last 15 years. Examples are the 2008 Axis Order Guidance policy (08-138r5)
> and later the http uri policy (in TC Policy Directives). Anyway, in 2013
> the WFS 1.1.1 corrigendum corrected the axis order issue to be consistent
> with the Axis Order policy as well as other OGC standards.
>
> Short synopsis: Before WFS 1.1.1 the only sure thing was that the axis
> order was LON/LAT when the notation "EPSG:4326" was used to define the CRS
> and otherwise was undefined.  For WFS 1.1.1 and later, the OGC Axis Order
> policy was in force which meant and still means that the axis order is
> defined by the CRS being referenced, such as lat/lon for EPSG 4326.
>
> The longer answers is:
>
> How any WFS instance interprets axis order depends on the version of the
> WFS standard implemented and the notation being used to specify the CRS.
>
> Based on the WMS 1.0 model, WFS 1.0.0 used the notation EPSG:XXXX for
> specifying CRSs.  The implication was (although it was never explicitly
> stated) that the WFS would do what WMS did and what the WMS did was to say
> that for EPSG:4326 the axis order was LON/LAT.  This was actually also
> consistent with GML 2.1.1. For example:
>
> <gml:Box srsName="http://www.opengis.net/gml/srs/epsg.xml#4326
> <https://urldefense.proofpoint.com/v2/url?u=http-3A__www.opengis.net_gml_srs_epsg.xml-234326&d=DQMFaQ&c=n6-cguzQvX_tUIrZOS_4Og&r=LCXhcrMdvdcRNz8NUgOEV1cYOakNFpC9sSTsSszC0Kc&m=HY5mDJJy4-L8dvfBgWuyrG9fxDiNEyFRZNZCetAE2kY&s=cz2ENba-ejcGrGosCxt7VCq5sYUHu7GlMCLq3G1jO2M&e=>
> ">
>          <gml:coordinates>-180.0,-90.0 180.0,90.0</gml:coordinates>
> </gml:Box>
>
> The WMS 1.0 standard did not say anything about any other EPSG code and so
> other CRS behavior was undefined.  As a result, most WFS 1.0.0
> implementations stuck with LON/LAT regardless of the EPSG code being used.
>
> WFS 1.1.0 specified several notations for defining the CRS: EPSG:XXXX,
> http://www.opengis.net/gml/src/epsg.xml#XXXX
> <https://urldefense.proofpoint.com/v2/url?u=http-3A__www.opengis.net_gml_src_epsg.xml-23XXXX&d=DQMFaQ&c=n6-cguzQvX_tUIrZOS_4Og&r=LCXhcrMdvdcRNz8NUgOEV1cYOakNFpC9sSTsSszC0Kc&m=HY5mDJJy4-L8dvfBgWuyrG9fxDiNEyFRZNZCetAE2kY&s=vcSZ-tEvudUXHYfE6HX2WUCkg5qjM9X5ULCLBSADUkw&e=>,
> urn:EPSG:geographicCRS:XXXX.  The EPSG:XXXX notation was maintained for
> backward compatibility with WFS 1.1. As such, a WFS implementation instance
> would behave as it did in WFS 1.0.0 (i.e. LON/LAT for EPSG:4326, undefined
> otherwise).  The other two notations were meant to follow the then being
> developed OGC Axis Order policy which eventually provided clear policy
> guidance on how to express and document axis order. The recommended option
> is to use the axis order as defined in the EPSG CRS definition.  However,
> at the time WFS 1.1 was being processed the Axis Order policy and notation
> were still in flux. Ultimately the URL and URN notations specified in the
> WFS 1.1 standard were invalid.  This problem was, however, addressed in the
> WFS 1.1.1 corrigendum. Unfortunately, for some reason that corrigendum was
> never published thereby increasing the WFS axis order confusion. This
> oversight is being addressed by OGC staff.
>
> Which brings us to WFS 1.1.1 and 2.0.X.  In these versions, the same Axis
> Order policy is specified: Do what the EPSG CRS definition says. Also, at
> the same time the srsName expression changed to the http uri notation (i.e.
> http://www.opengis.net/dev/crs/epsg/0/xxxx
> <https://urldefense.proofpoint.com/v2/url?u=http-3A__www.opengis.net_dev_crs_epsg_0_xxxx&d=DQMFaQ&c=n6-cguzQvX_tUIrZOS_4Og&r=LCXhcrMdvdcRNz8NUgOEV1cYOakNFpC9sSTsSszC0Kc&m=HY5mDJJy4-L8dvfBgWuyrG9fxDiNEyFRZNZCetAE2kY&s=otM5bDnCQOb9DY-U_pSooY0tHUmXzTpcwfIOXZvwxyk&e=>).
> This is because in 2010 the OGC policy changed regarding how to identify
> resources. The OGC moved from urn's to using URLs as identifiers. Even so,
> the initial version of WFS 2.0 used the URN notation
> (urn:ogc:def:crs:EPSG:<ver>:XXXX). This changed in WFS 2.0.2
> (corrigendum). All of the urn references were changed to URLs with notes on
> what the older "urn" strings were.
> Which brings us to the GeoServer documentation. That documentation is not
> quite right and should be corrected to reflect reality and what is actually
> in the various versions of the WFS standards. Further, the GeoServer
> documentation mentions the use of "urn:x-". I am not sure where that is
> coming from as none of the WFS standards from WFS 1.0 and later use that
> notation.
>
> Regards
>
> Carl
> --
> Carl Reed, PhD
> Carl Reed and Associates
>
> Mobile: 970-402-0284
>
> "When the power of love overcomes the love of power the world will know
> peace." Jimi Hendrix
>
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